OLER, N. On Subgroups of the First Kind. MATHEMATICA SCANDINAVICA, [S. l.], v. 20, p. 91–92, 1967. DOI: 10.7146/math.scand.a-10824. Disponível em: https://tidsskrift.dk/math/article/view/10824. Acesso em: 14 aug. 2026.